Method of generating an accurately focused beam of charged particles



Aug. 26, 1 952 R. J. VAN DE GRAAFF ET AL 2,608,664 METHOD OF GENERATING AN ACCURATELY FOCUSED BEAM OF CHARGED PARTICLES Original Filed Sept. 18, 1945 3 Sheets-Sheet 1 /5 Invaders.

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METHOD OF GENERATING AN ACCURATELY FOCUSED BEAM OF CHARGED PARTICLES Original Filed Sept. 18, 1945 3 Sheets-Sheet 2 E, k? 8 e0 12171821330215. RoZuariJ Vandefiraqfi' Wdliam'weilez'fiuechrzer Aug. 26, 19 R. J. VAN DE GRAAFF ETAL 2,608,664

METHOD OF GENERATING AN ACCURATELY FOCUSED BEAM 0E CHARGED PARTICLES Original Filed Sept. 18, 1945 3 Sheets-Sheet 5 VOL TAGE A S0ufc OF A L TERNA TING YGL TA 6:

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Patented Aug. 26, 1952 7 UNITED STATES PATENT OFFICE METHOD OF GENERATIN AN ACCURATELY rocusao BEAM 0F CHARGED PARTICLES Robert J. Van de Graaff, Belmont, and William- Weber Buechner, Arlington, Mass, assignorsto ResearchCorporation, New York, N. Y., a corporation oil New York Original application September 18, 1945, Serial l lo. 617,036. Divided and this application November 18, 1949, Serial No, 128,084

14 Claims. (01. 313-4),

This application is a division. of our col-pending applicaticnser. No. 617,036, filed September 18, l945,-itjow Patent No. 2,517,269, dated August 1', 1950, and is directed solely to the -method of generating an accurately focused beam of charged particles disclosed therein, and" isF-divid'eci out therelrorn a separate and distinct invention.

This invention relates to almethod of generating an accurately focused beam of charged particles.

In order that the broad principle: of such invention may be readily understood, we have in the accompanying drawings disclosed two embodiments of apparatus by which our said method may be practiced, such method in the very nature of things necessitating apparatus for practicing it, but existing as a method claimable as such, irrespective of the particular apparatus for prac ticing it. i

The said method involves the employment of a highwoltage hlg'il -VZLCUUIH tube for producing accurately focused beam of swift particles, either electrons orlions. One means for producing 'a suitable source of high-voltage high-vacuum tube is an electrostatic generator of the type described in the=United-States prior patents to one of us, l l'o .l,99l,236, dated February 12, 1935,

and -No, 2,230,473, dated February 4, 1941.

The-in vention makes possible an improvement in highwoltage radiography and may be carried out in or by the use of high-voltage high-vacuum tubes involves new principles of operation. The invention'may be useful in such fields as nuclear physics, cancer therapy, radiography, high-voltage X-rays, the rectification of highvoltage currents, the production of cathode rays; the sterilization of foods, drugs and other substances, the irradiation of matter, and the acceleration of electronsfor high-voltage electron microscopes. l

Our invention herein claimed may be defined as a method of establishing, maintainingand transmitting a continuous, uninterrupted, accurately-focused beam of chargedparticles in a substantially uniform, high-voltage high-vacuum, electrostatic fi'eldwhich comprises the following stepsfl (1') establishing and uninterruptedly main-- taining a substantially uniform, high-voltage, high-vacuum, electrostatic field in'a suitable envelope, (2) establishing,uninterruptedly main taining and uninterruptedl-y transmitting a conation of the hlgh voltage high-vacuum substantially uniform electrostatic field; (3,); by the uninw terrupted maintenance of such substantially uniform electrostatic field, accelerating such continuous beam of charged particles in its passa e from such emission area onwardthroughout such substantially uniform, electrostatic field to such electrode target, and (i4) accurately focusing Said continuous beam of charged particles throughout its passage from such emission area to; such electrode target, l

The. high-voltage high-vacuum tubev to which our present invention of a novel method relate and by which it is preferably practiced, is a 00.n-

' stant potential X-ray tube of the order of two million volts, suitable for operation eithersealed off or with a continuouslyoperated vacuum pump. Without limiting our invention to the use of such apparatus in practicing our method, which maybe practiced by the employment of anyothe suitable apparatus: (though no other antecedent to that herein. disclosed is knownto, us), We will describe in d'etai'l novel apparatus for the purpose, but'first we state that other, substantially dinerent structures of apparatusby which our herein claimedmethodmay be practiced are disclosed in our co-pending application, Serial No. 297,036; filed July 3, 1952, entitled High-Noltage Apparatus for Creating a Substantially Uniform Eleca trostatic Field for the Acceleration and Delivery of Charged Particles all with. the Same Generae torEnergy.

Such apparatus: as herein disclosed comprises a highwoltage tube of insulating materialwhich has electrodes adapted to be connected to a suitable source of high potential, such as a high-1 voltage electrostatic generator. At one end of the tube, being the upper endas shown in the drawings, is located means providing an emitting source, which means, in the present disclosure, is afilament having a plane emitting surface of relatively minute area. As will be subsequently described indetail", the wall of thetube comprises electrode rings owing-like disksor centrally open metallic diaphragms arranged along the tube, spaced by insulationsuch as glass, and connected in suitable manner to. the corresponding elem trodesina high-voltage generator insuch a way that the potential'gradient downthe tube is-uni'- 'tinuous-beam of chargedpar ticles froma source ofchar ged particles at one end of such envelope to an 'electrod'etarget'at the' other end of said envelopethrough such high-voltage for the operform, and in such a way that equal-stepsin the voltage between successive electrodes are-pro vided. Thuathere is provided in the tube s, substantially uniform electrostatic field.

An exceedingly important feature of the invention with respect tothe novel rn'ethod is the 3 provision and use of a substantially uniform electrostatic field which is in contrast with the use of a non-uniform electrostatic field, as has generally been the case in tubes that have been used in the past, as, for example, the tube disclosed in the United States patent to John G. Trump, No. 2,182,185, dated December 5, 1939.

The focusing of the electron beam by the herein disclosed apparatus for practicing the invention, and by the use of a substantially uniform electrostatic field, is less sensitive to variations in the potential applied to the various tube electrodes than is the case in tubes employing nonuniform electrostatic fields.

One advantage resulting from the use of a substantially uniform electrostatic field, in combination with a magnetic lens in a high-voltage vacuum tube, is that thereby extremely fine focusing is obtained of a high-speed electron beam of the order of two million volts. Moreover, it appears that the construction referred to, one embodiment of which is herein disclosed for practicing our method, being simpler, is more reliable than'prior constructions. The employment of a substantiallyuniform electrostatic field is materially associated with breaking up the voltage along the tube into very small divisions, which is also desirable from the point of view of insulating very high voltage.

With a uniform electrostatic field substantially the entire cross section of the high voltage vacuum tube can be used for the acceleration of the charged particles, whereas, in a non-uniform electrostatic field, as heretofore generally employed, the region which is usable for focusing is usually very close to the axis and is most effective only for the paraxial rays. Thus, with the use of a uniform electrostatic field, the electrons may be suitably accelerated through a region whose cross-sectional diameter is relatively large when compared with the length of the high-voltage tube. A further advantage of the electrode configuration used, which breaks upthe voltage asjust stated, is that such configuration also minimizes the effect of any uncontrolled electric charges on the inner surface of the wall of the insulating material of the high voltage tube;

As'far as we are aware a uniform electrostatic field alone has never been used inhighvoltage vacuum tubes for accurate focusing prior to the present invention.

When substantially the entire acceleration of the electrons is done in a uniform electric field, there-is made a full and direct use of the longitudinal component of the electric field, with a minimum interplay or even presence of the transverse component of the electric field, a component in itself useless for the acceleration of the ions in the desired direction.

Heretofore in attempting to focus the electron beam, means have been used constituting a relatively complicated guidance or compulsion. It appears that, both in theory and in practice, if the electrons are permitted to fall in or be accelerated by a simple uniform electrostatic field, the result is more satisfactory than the result obtained with more complicated means, the elements whereof require a certain definite, simultaneous adjustment relative to each other.

In some prior high-voltage tubes a part only of the tube had asubstantially uniform electrostatic field, but in'all such cases known to us the part of the field of such tubes that was nonuniform in characterwas actuallythe party that was the most important of all as regards direct- 4 ing the motions of the charged particles. Thus, where in prior instances, a uniform electrostatic field was created in part of a high-voltage tube, o, it was not primarily for. the purpose of focusing a beam, but mainly to simplify other features of the construction. An instance thereof is shown in the patent to Trump, No. 2,182,185, above referred to.

In certain other tubes of the prior art the very beginning of the path of the electrons or ions was not in a, uniform field and was actually sharply distorted, so that there resulted an initial spontaneous breakdown creating a localized source of ionization by virtue of the fact that the electrostatic field was extremely non-uniform in character. Also, in such instances in the prior art, tubes were made for operation with impulses where the voltage was on for periods of the order of only a few microseconds each, and in order to pass sufiicient average currents they had to have high instantaneous currents.

The momentary breakdown in the tube afforded extremely high instantaneous currents, so high that the accompanying space charge would tend to distort, during the moment of actual operation of the tube, the uniformity of the electric field in regions which had been uniform just previous to the discharge.

In the apparatus herein disclosed as a representative embodiment of means for practicing our method, in order to provide a path for the electrons or charged particles through the high voltage tube, preferably a two-inch diameter hole is cut out or is otherwise provided at the center of each of the metallic ring-like electrode disks or diaphragms provided along the extent of the tube. Since each successive electrode ring or diaphragm is more and more positive from the filament toward the target, the electrons or negatively charged particles are attracted down the tube and strike the target with an energy corresponding to the full generator voltage. The conditions are reversed when positive ions are to be accelerated. In their passage down the tube, they tend to follow the lines of electric force, and in the high-voltage vacuum tube herein disclosed, the lines of force are straight lines. Consequently when the electrons or charged particles reach the bottom of the tube and strike the target, they are all traveling in parallel paths. and all have the same energy. Such a result couldhardly be secured where an alternating current device, such as a transformer, is used for the voltage source, because in such case the electrons usually have all energies ranging from some indeterminate low value up to that corresponding to the peak of the alternating current wave.

There is thus an essential difference between direct current equipment where all the electrons striking the target have the full generator energy, and all alternating current equipment where only a few of the electrons have the full rated peak voltage,- the remainder being of lower energy.

The structure herein disclosed is equally well suited for the acceleration of either'positive ions or negatively charged particles. This follows since themanner of construction and the use make the tube completely symmetric. Thus, it is possible to accelerate charges in either direction through the tube without the necessity of having to change the arrangement of potentials on theelectrodes. The electrons are emitted at-the negative endof the, tube and are accelerated toward the electron-collecting target; while atthe same timepositive ions are to be produced at the positive end of the tube and accelerated toward the region of the cathode.

Other things being equal, the" diameter'of the beam of charged particles after passingthrough the tube is proportional to the size of the source of the charged particles. When the method is practiced in or by an X-ray tube for radiography, the definition in the radiograph depends critically upon the spotsize, and hence it is very desirable that the efiective portion of the filament be as small as possible. As more fully'set forth in'the description of the drawings, the focused spot upon the target can be smaller than 0.01 of aninch in'diameter. .To obtain radiographs of thick. sections having good definitionpthe: size. of the. focal spot mustbe veryfsmall so that the. X-rays will be emanating from apoint source. Thick metallic sections of objects requiring on the order of. two' million volt X-rays presentnew geometric problems making essential the use of suchsize of focal spot.- As stated, the high voltage tube operates in conjunction with an. electrostatic. gene erator producinga potentialof theorder of two million volts. The use of such constantpotential hasbeen found necessary in order to obtain and to maintain the said extremely fine focusing referred to and to provide optimum conditions for heat dissipation at the focal point. As subsequently set forth in detail, the target upon which the electron beam is focused is a thick disk of gold used in association with high pressure water cooling.- Theuse of such a relatively thick target disk permits operation with the target spot in molten condition without, however, melting entirely through the. disk. It becomes possible as. a result to make full use of the high intensity, sharply concentrated, electron beam andthus to obtain X-ray pictures of greatly improved quality. l i

. The invention will be better understood in detail by reference to the following'descriptionwhen taken in connection with the accompanying illustration of apparatus or means by. which our method may be practiced, while the scope of the novel method will be moreparticularly pointed out in the appendedclaims. a. sin the. drawings: l 1' is a. vertical or longitudinalmcentral', cross section of a high voltage vacuum tubeby means. orwhich the method. herein disclosed maybe practiced; 1

Fig. 2 is atransverse or cross-section upon the line 2-2.of Fig.1; 1

Fig.3 isa detail in, vertical central section through the lower end of the filament? and the surrounding guard ring; i Fig. 4. is a view similar to Fig. 3 but on arlarger scale and representing only a portion of the guardring; l

Fig. 5 is abroken-away detail inside elevation of a portionrof the high-voltage vacuum tube shown inFig. 1,with' a diagrammatic indication of the connections between the electrode rings .oflthewtube and corresponding electrodes of'an electrostatic generator; l .2: (dis :a. broken-away detail in vertical sec.- .ltionlcf .azmodified form of the electrode: rings of thetshigh voltageatube at thalower portion of saidwtube; and}- l v Figs 7v is alvertical crossesectional view; of. a

high voltage rectifier thatoperates in accordance representingzone of the most important. aspects,

of the invention. r

Referring to, the drawings illustrating apparatus by which our novel methodmay ,be practiced, therein is shown a high-voltage vacuum, tube consisting of a column composed of; glass rings and of metal electrode rings or ring-like diaphragms or disks suitably welded together in alternation throughout the column, a. part only of which is shown, in a manner and by means not horeinlnecessary to disclose in detail, the present invention being directed broadly to: the herein disclosed method by which a. uniform electrostatic fieldis providedand maintained inthetube or column, whereby in some applications, when the method is practiced by an apparatus usedas an X-ray tube, the electron. beam is controlled and compelled to strike the target. at. apoint. of, exceedingly small. diameter.

This configuration of electric field is. also well suitedfor the acceleration, and focusing of ion beams.

In Fig. 1, the glass rings are respectivelyindicated at. l, and the metal. electrode ringa'cen- *trally; open diaphragms or disks at 2. The. said metal rings 2 or the like. are. electrode rings'and lie accurately. placed in planes perpendicular; to the axisofthe tube, and they are placed atequal distances apart, as, for example, one-third of an inchin the present disclosure. In Fig. :1, thetube or column is represented as broken away be cause of the necessity of presenting a; viewer the complete tube in a single figure; While, obvious:- ly the method is not concerned with any particular size or proportion of parts of thesap paratus by which the method is. practiced, it is ointed out that in the. illustrated embodiment of the high-voltage tube the distance in the actual structure from the horizontal line. 3,to. the boilzontal'line dais fifty-seven inches. the diameter of the opening in each ring is twoinches, andthe outside diameter of the-tube. or column is three. inches. As clearly shown in Fig. 3, the outerredge of each of the metal electrode rings 2 is substantially co-terminous with the outer edge. of the glass rings i. In the simplified formof, highvoltage tube,represented in Fig.1, the distance from the line 3 to the top ofthe dome-like. glass insulation is aboutsiX inches. As stated, however, these dimensions may be varied asjfound suitable, and the scope; and principle of our method are in no wiserestricted by this recitae tion of dimensions of apparatus. for practicing ourmethod.

" Intheplane of the top metal electrode'ring l. a metal disk 5-is provided which substantially fills the opening inside said topmost metalring 2.

which disk 5 is maintained at the same potential asthe top metal'electrode 2. This insures that the electric field immediately below the region of the disk 5 is uniform. The glass insulation which holdsthemetal disks 2 in correctrelative alignment and which consists of the glassrings i. may have on its inner surface an uncontrolled distribution of electric charge which wouldtend to'distort in a random and uncontrolled manner the uniformity of the electrostatic field within the main region of the tube. However, the distributing :infiuence of these charges is reduced to. a negligible' degreeby the shielding effectiof. the metal rings 2,which.extend inward from-the glass wall composed of theglass rings I towardtheaxis of the tube to a sufficient extent to produce the desired shielding. The fact that thegap. between adjacentmetal rings 2 is relatively small,

being in the present disclosure one-third of an inch less the thickness of one disk, the actual structure having the other proportions above specified, makes it possible to obtain the desired shielding effect with only a relatively, narrow ,region or portion of each metal ring extending inwardly beyond the inner surface of the glass wall composed of the multiplicity of glass rings I.

In the. disclosed: embodiment of apparatus by which our method may be practiced, the amount that each metal ring must project inward from the glass wall of the tube must be approximately thesame as thelength of the gap between next adjacent metal rings 2. all along the glass wall of the tube. Thus, the fact that in the disclosed embodiment of apparatus for practicing our method the gap between the next adjacent metal rings 2 is small is in itself advantageous, inasmuch as it reduces the amount that each metal ring 2 must extend inward beyond the inner surface of the glass wall. 7

In order to obtain a uniform electrostatic field, whichis an exceedingly important stepin our novel method, it is essential that the metal rings 2 be close together. Thefact that they are placed closet'ogether makes it possible to insulate a high voltage per unit length of the tube.

Moreover, the fact that the metal rings 2 are close together makes it possible to use more of the internal space in the tube for the beam of charged particles.

Certain metal rings 2 of the tube or column, which are indicated at 2a in Fig. 5, are connected to corresponding electrodes of the generating apparatus which may take the form of a high voltage electrostatic generator, as indicated in the diagrammatic part of Fig. in such a way that the voltage between the successive electrodes of the tube is the same.

' In Fig. 5 a few of the generator electrodes are represented at 21'), and a portion of the resistors at 20. As shown, every'third electrode ring 2 of the tube is connectedito a corresponding electrode of the generator, which generator electrodes are an inch apart. Each of the metal electrode rings 2 in Fig. 5, as well as in Fig. 1, has its outer edge substantially co-terminous with the outer edge of each of the glass rings I.

- The simplicity of the described arrangement and the useful results flowing therefrom may, to

be appreciated, be compared to optical problems in'photographic cameras. Heretofore, in the ordinary type of tube for the general purpose of apparatus herein shown, the electrical focusing was done by a series'of what may be termed strong lenseswhich must have a very definite and specific relation to each other in order to produce the desired focusing action, and maybe considered analogous to an optical system of a photo- To continue the analogy, where in a "photo- :graphi'c camera a series of strong lehses is re- 8 quired which musthave a definite relationship, any error in the strength of the lenses can upset such definite relationship. In a photographic camera the stronger the lens is the more accurate such lens should be, and the more accurate should be its relation to all the other lenses of the camera. i

As has been stated in the foregoing, the acceleration of an electron beam in a uniform field has many basic advantages as contrasted with the more usual methods of acceleration in strongly non-uniform electric fields. However, it may be. desirable while still using a substantially uniform electric field for acceleration to modify it or warp it slightly, for example, in dealing with certain practical situations which would not arise in an entirely ideal case. In order to overcome the spreading effect, due to the space charge of a positive ion beam, itmight be desirable to have the top part of the accelerating electric field slightly converging. This condition could be realized simply by having the voltage difference from electrode to electrode constant inthe lower and middle portion of the tube, but with this voltage difference slightly decreasing as the very top of the tube is approached.

Referring to the use of the apparatus as an X- ray tube in the practice of our novel method, the filament of the tube from which emanates the electron beam is indicated at ID in Fig. 1, and is shown in detail in Figs. 3 and 4. The said filament is composed of tungsten, and is of a hairpin type. It has the apex of the bend ground off, as indicated at H in Figs. 3 and 4, in order to provide a plane emitting surface ll of relatively minute area. The diameter of the filament in the unreduced portion thereof is 0.010, and at the ground-off portion at the apex of the bend it is desirably less than one-half such thickness, thereby insuring an intense heat at said groundoff portion when the apparatus is in use, being the plane emitting surface of the electrons. The cross section of the filament being the least at the ground-off portion, the resistance is the greatest at that area.

The filament [0 has placed in conjunction therewith and encircling the same, a guard ring [2, shown enlarged andzin part injFig'.'4,'which has a plane lower surface lying exactly in' the same plane as the emitting plane'of the filament. The said guard. ring 12 hastherein'a central through-opening 12a, which is approximately 0.040 of an inch in diameter and within which the apex of the bend, constituting the plane emitting surface II, is symmetrically positioned.

The filament and the surrounding guard ring are usually maintained at approximately the same potential. However, by making the potential of the guard ring substantially more negative than that of the filament,'the gridaction of the guard ring can be used to reduce, or even entirely cutoff, the electron stream. For this/purpose there are shown in Fig. 4 wires 10a and 12b leading respectivelyv from the filament Ill and from the guard ring l2. to the positive and negative sides of a. battery 13. Also the over-all focusing properties of the tube as a whole may be affected by providing relatively small'voltage differences between the filament and. the surroundingguard ring. Ihus, although the filament andlthe guard ring have been generally'operate'd at the same potential, there are some occasions when itis desirable to operate the filament and guard ring at somewhat'differentpotentials. By reason of the plane emitting'surface ll of aeoaeea the filament II) and of the uniform field within the tube column, the .beam of electrons proceeds in a substantially straight line along the tube or column "from the point of emission, as indicated at I3, resulting in a beam whose cross section in the region nearthe top of the tube corresponds closely to theisize and shape of the emitting plane, and wherein the energy of the individual charged particles is substantially identical. Such a beam may readily be focused by a relativelyweak magnetic field on an extremely concentrated spot, as by an electric magnet I4, the arrangement constituting a magnetic lens, the magnetic lines of force whereof are indicated at Ma.

Where the apparatus in the practice of our method is used for generating X-rays, as for high voltage radiography, the electron beam is focused on a target which, in the disclosed embodiment of apparatus for practicing our method, is a thick metal disk I5 of gold, used in association with a high pressure water-cooling jacket, indicated at I6, and provided with a water inlet I1 and water outlet I8. The target I5 is a gold disk one-quarter of an inch in thickness in the disclosed apparatus for practicing our method.

With the usual construction for X-ray targets, a high voltage beam of electrons of great concentration would melt locally the target employed in such construction, and thus cause leakage of the cooling water in the vacuum of the X-ray tube, or cause cracking of the tungsten target and impair its usefulness. This would prevent further use of any suchdevice until repaired. However, with'a thick target of material such as gold, which has a high melting point and high heat conductivity, the molten region is small, and since it does not extend entirely through the target, no leak is caused. The surface tension of the liquid gold tends to keep the gold fromfiow me away. It is observed in practice that the vapor pressure of the liquid gold is so low, under the operating conditions in carrying out our method, that the thinning of the target due to evaporation is negligible.

, Although tungsten has generally been used as a standard material for targets, experience with the gold target herein disclosed indicates certain advantages. Gold has a high heat conductivity and also chemical and physical properties such thatit can be repeatedly melted and allowed to freeze without appreciable oxidation or change in physical structure.

The high voltage vacuum tube herein illustrated and described constitutes one embodiment only of apparatus for practicing our novel method, and presents the same in a simplified mechanical construction. Certai-n mechanical design features of the tube herein disclosed, such as areconcerned with a vacuum-tight tube or column having the necessary insulating characteristics and enabling it to withstand the high pressures involved, as, for example, an air pressure of from 200 to 400 pounds per square inch, have, in the further development of our novel method herein described and'claimed, been ably developed by Machlett Laboratories, Incorporated, to whichhas issued United-States Patent/No. 2376,4351, dated May 22, 1945, -in the names of "Raymond R. Machlett and Joseph W. Skehan, bu-t'not claiming subjectsmatter herein claimed, and such matters of mechanical construction are, of course, not herein claimed. The invention herein disclosed and claimed is particularly directed to establishin and maintaining a uniform electrostatic field for accelerating and focusing charged particles, and to carry out such method we provide an arrangement of electrodes particularly suitable for establishing andmaintaining such an electric field. As a part of the apparatus, but which is not herein claimed, we provide a filament of a very small effective area and a relatively thick gold target. The described construction has the additional advantage of shielding the insulating walls of the tube, and breaking up the total applied voltage so as to facilitate problems of insulation.

The high-voltage vacuum tube herein disclosed for practicing our novel method is adapted to the acceleration and focusing of charged particles, andin the case of electrons this beam isextremely concentrated. The disclosure includes charged particle accelerating means providing a uniform accelerating field, thus, reducing to a minimum the dispersion of the charged particles throughout their travel. Therefore, a large number of accelerating sections are provided, the number used inpresent practice for two million volts being approximately 180, thereby providing uniform accelerating steps of-l2,000 volts each.

Thus, in such embodiment of the disclosure and referring again to Fig. 5, the electrode disks welded into the wall of the tube in alternation with theinsulating disks to constitute the entire wall of the tube comprise sixty groups of metallic electrodes, each such group consisting of three electrodes, and only one of said electrodes of which each such group is composed is directly electrically connected to a corresponding electrode of a high-voltage electrostatic generator, so that the voltage between the successive disks of the tube is the same.

In Fig. 6, there is represented upon an enlarged scale a slight modification of the shape or form of certain of the electrode rings indicated at 2 in Figs. 1 and 5. In Fig. 6 there is shown in cross section a small portion only of the lower part of the tube. Therein the glass rings are indicated at I, as in Figs. 1 and 5. The electrode rings are indicated at I9, and each of them has a much thickened inwardly extending portion 20 of streamlined form and constituting what may be termed .a .tear drop section. Such shape is evident from the cross section View. In reality each portion 28 is of annular form having extending outwardly therefrom the thin .rimlike part I9, thus constituting an annular flange received between the next adjacent glass rings I. Electrode rings of tear drop shape are employed only at the lower end of that part of the tube composed of alternating glass rings and metal rings.

Under certain conditions such tear drop construction of electrodes may have advantages including the following: (1') improved electrostatic shielding of the inner wall of the insulating envelope of the tube; (2) increased shielding of the electron beam inside the tube against the disturbing eifect of irregular discharges on the glass wall of the tube; (3) by constructing the internal thick electrode of some suitable magnetic material, such as soft iron, the electron beam can be shielded with greater effectiveness from disturbing external magnetic fields; (4) in certain special cases where the electric field may locally depart from the uniform condition, as, for example, at the lower end of the vacuum tube shown in Fig. 1, the rounding of the electrodes would prevent the concentration of the electric field which occurs with the use of ring-plane electrodes 2 in this local region.

Another advantage of the use in accordance with our novel method, of a uniform electric field for the acceleration of charged particles may be noted here. For a given voltage and a given length of tube, such method limits to a minimum the values of the electricintensity or stress in the tube. In the herein disclosed method for producing an acceptably focused beam of charged particles at high voltage and apparatus for practicing such method, the maximum potential gradient at the surface of the electrodes is held to a minimum and'approaches as a lower limit the value of the ratio of the voltage to the length of the tube. Thus, other things being equal, the maximum potential gradient at the surface of the tube electrodes in practicing our method is reduced as compared with other constructions, making it easier to insulate high voltages. The fact that the potential gradients are reduced to a minimum makes it possible to apply to the tube its rated voltage even though the vacuum region of the tube may be temporarily filled with gas at the pressure surrounding the outer portion of the tube. This may be desirable for certain special tests, or when two or more tubes are connected in parallel with the same voltage source. In case of vacuum difficulty with one of the tubes, it could be allowed to fill with gas so that the other tube may continue to be used at full voltage.

The substantially uniform, electrostatic field provided throughout the transverse and longitudinal extent of the enclosed space that is created and is undisturbed and undistorted, in accordance with andin carrying out the herein disclosed method, cooperates and co-acts, as stated, in focusing the beam of charged swift particles, and also cooperates and co-acts in effecting the reaching of higher voltages, and provides for carrying out the herein disclosed method by apparatus of greater compactness and materially differing in structure among themselves.

The region along the axis of the high-voltage enclosed space (herein shown as an accelerating tube) is left open and unobstructed so that the desired beam of charged swift particles can be and is accelerated by our method along the axis of such enclosed space, that is, along the axis of the insulating portion of the said tube.

However, if the voltage be sufiiciently increased, a spontaneous and regenerative high-voltage discharge occurs along this open region of such enclosed space (the herein disclosed tube), thus limiting the voltage which can be successfully applied to such enclosed space. The nature of such regenerative process or action is not as yet completely understood by scientists.

With the herein disclosed and claimed uniform electrostatic-field method, the potential gradient can be kept at relatively very low value at electrode surfaces or areas in vacuum existent in or at the wall of the said enclosed space or tube (the form or structure of which may differ in variousapparatus carrying out our method), particularly in the region adjacent to the open axial path through the tube or enclosed space. This relatively very low value of the potential gradient can be made to approach the theoretical limit which is the ratio of the voltage across the tube or enclosed space to the length thereof, which may be called the average potential gradient along the tube. The low value of the potential gradient at electrode surfaces or areas, or surfaces at controlled potentials, at or upon the wall of the enclosed space is important because it reduces the auto-emission, or field currents, of electrons at these surfaces or areas. It is also important because it reduces the secondary emission of 7 charged particles due to the impact of stray highvoltage particles on such electrode surfaces or areas or surfaces at controlled potentials.- Secondary emission is known to be a most important factor in the breakdown of high-voltage tubes.

In a uniform, electrostatic-field enclosed space or tube, the impact of stray high-voltage particles with its electrode surfaces or areas or surfaces at controlled potentials, tends to be in a direction perpendicular to those electrode surfaces or. areas, thus holding to a minimum the production of secondary particles contributing to high-voltage breakdown. It is important to avoid impacts at glancing angles upon such electrode surfaces or areas since these produce a large number 'of the undesirable secondaries. i T

In the uniform, electrostatic-field enclosed space or tube, when secondaries are produced by impact of stray high-voltage primary particles, the'secondaries then move along lines parallel to the axis of the enclosed space or tube. This is in contrast with the situation in non-uniform-field high-voltage tubes where, in general, these secondaries move in various directions and along relatively complicated paths. The fact that with the uniformelectrostatic-field enclosed space or field tube the secondaries move in directions parallel with the tube axis, makes it feasible to deal with them in such a way that they are prevented from building up the cumulative breakdown.

The principle of the substantially uniform elec: trostatic field is applied primarily to the axial region of the enclosed space or tube where the desired beam of charged swift particles is accelerated by the herein disclosed method, and also to the electrode surfaces of the wall of the enclosed space or tube in the region adjacent to and surrounding such axial region. The electrode surfaces at the wall of the enclosed space or tube having high voltage, tend to contribute secondary particles which may cause high-voltage breakdown, and as herein disclosed we shield the Wall from deterioration due to stray radiation. Summarizing with respect to the step of accelerating the beam of charged swift particles through the creation of a substantially uniform electrostatic field into which such particles are emitted, it will be evident that low-energy secondary particles that may be emitted from electrode surfaces at the wall of the enclosed space or envelope (wherein we create a substantially uniform electrostatic field), in the region adjace t to the lo itudinal axis of the enclosed space or envelope, are accelerated in a direction substantially parallel to the longitudinal axis of such enclosed space or envelope. Also, the impact of stray-high-voltage particles with electrode sur-' faces at the wall of the enclosed space or envelope is largely in the normal direction, thus minimizing the production of secondaries, and furthermore, the potential gradients on the electrode surfaces exposed-to the bombardment of stray high-voltage particles do not substantially exceed a value equal to the ratio of the enclosed space or tube voltage to the enclosed space or tube length, and, moreover, the potential gradients on the electrode surfaces at the wall of the enclosed space or envelope do not substantially exceed the average potential gradient along the tube or enclosed space or envelope.

In the apparatus herein disclosed for practicing our method, we emphasize that the multiplicity of metal electrode rings orconducting electrode members 2 are for the purpose primarily of carrying out the step of our novel method, namely, that of establishing a substantially-uniform elec- 13 trostatic field throughout the said tube, and in doingso we do not extend the outer edges of said members materially beyond, but, on the contrary, keep their outer edges substantially flush with, the outer edges of the insulating members I, and we avoid any construction which would cause, within the tube structure, any considerable amount of stored electric energy, as this would prevent the carrying out of our method. We also avoid anyconstruction that would provide appreciable capacitance between adjacent conducting electrode members 2, as this would prevent the carrying out of ourmethod.

In Fig. 7 is sufiiciently representedian improved high-voltage rectifier possessing, many of the principles set forth in the foregoing part of the specification, and which presents one of the most important aspects of the invention herein disclosed and by which also our novel method is practiced. Such rectifier is shown in its relation to a source ofalternatingvoltage of about a hundred volts.

. Therein is shown in vertical cross section a view of such a high voltage rectifier, the interior of which is. highly evacuated. In said figure the emitter or cathode, being an approximately plane electron-emitting cathode, is indicated at 2| at the upper end thereof, (and the. anode is indicated. at 22 at the lower end thereof. ,The cathode could be constructed by providinga large number of closely spaced, parallel wires heated by the passage of an electric current.- i i .The outer, herein vertical, wallor main cylindricalportion of the rectifieris preferably composed, as in Fig l, of alternate insulating rings 23 preferably of glass, ancl'goi'tmetal electrodes 24,-which may be simila-r to those shown at 2 in Figl l and possess the characteristics, purposes andadvantages thereof, r

Jackets for water cooling are provided at the upper and lower ends of the rectifier, as indicated at 25 and 26, and theouterrsur-face of the main cylindrical portion of the rectifier will, if necessary, be cooled by circulating oil or gas.

The electron stream, so marked, is indicatedat Z1, and theresiston at 28,rwhich is indicative of a suitable" generator, such as would be employed with the apparatus shown inFig. l. .A source of alternating voltage,.such as a transformenis diagrammatically indicated at 29 in proper relation to the high voltage rectifier The 'multi-sectioned construction, one embodi mentlof which is herein disclosed,.makespossible the satisfactory. insulation of extremely high inverse voltages, whil-esome of the principles-of electron flow, as described in the preceding part of the specification, with relation to X-ray tubes, together with other. aspects of the construction, make possible the flow, or large electron currents inthe .desireddirection in thecarrying out of our novelmethod. This is supported by the fact thatiin tests with X-ray tubes of the type described in the f-oregoingpartof the-specification, well-focused electron beams have been obtained at1.-surprisingly low voltages. I J -r In the .multi-section-edhigh voltage evacuated rectifier herein disclosed, the electrons carrying the desired current move inpaths substantially parallel They travel in a substantially. :uniform electric v,fie'ldtdurin'g the whole of their passage from cathode to anode. The electrons move from cathode to anode through a region in which the electric component perpendicular to the main axis of the tubeor rectifier is .zero.

M In such a multi-sectioned high-voltage evacu- 7 l4 ated rectifier, the maximum potential gradient on the electrodes 1 in vacuum is a minimum, and never exceeds a value approximately equalto the ratio of the maximum inverse voltageacross the rectifier to the length ofthe electron path within the rectifier. i While there is hereinshown and describedone specific means for practicing our novel method and for carryinginto effect the principles which have been set forth, tog-ether with .a slight modi fication of such specific means, it is to be. understood that our disclosed method involves estab lishingand maintaining a substantially uniform electrostatic field wherethrough passeswan accuratelyi-ocused ibeamxof charged or swift aparticles, either electrons or ions. The specific terms herein employed are used in "a generic and descriptive sense and not for purposes of limitation. I

Whatweclaimis: i

1. That method of effecting the generation of and the acceleration and delivery of antaccuraltei- 1y focused beam of charged particles; Jeither electrons or ions, for use in fieldsof nuclear physics, cancer, therapy, radiography, high-voltage X-rays, cathode rays, etc., through the "per:- forming of a series of steps involving the create ing and injectingof a beam of charged particles directly and immediately into a substantiallyuniform, steady and continuous electrostatic field for the purpose of securing the delivery of uch beam of particles as a bee-moi charged swift, accelera'ted particles throughout the length of such substantially uniform electrostatic field to the point of use thereof, comprising the following steps; (1) .first,removing the gas from within an else;-

tronic envelope tosuch an extent asto create and maintain an extremely high vacuum therein; (2) then, creating and maintaining within's'aid vacuum in such electronic envelope throughout the transverse area and lengthwise extent an'd parallel with the wall thereof, a substantially -uni form electrostatic field that is undisturbedand undistorted along the axis of such envelope and for the passage along such axisof such beam of charged swift, accelerated particles} and .(3), next, creating and injecting directly and. immediately into said substantially uniform electro static field such beam of charged particles toward an opposite point of use thereof, whereby such beam of charged particles so injected directly and immediately into said field accelerated through said field along the longitudinal axis of such electronic envelope and is deliveredlas an accurately focused beam of charged swift, ,accel: era-ted particles to the point of use at the 'ter mination of such electronic envelope, oppositthe area of injection thereof, withjall such charged swift, accelerated particles traveling in parallel paths and all with the same 'generator 'energy.

2. That method of directing, accelerating and focusing a beam of charged particles, either electrons or ions, from a point of injection-thereof so that they are caused to travel invery close parallel paths 'to the point of use thereonallhaving the same energy, comprisingth-e following steps: ,(1) providing and maintaining averyhigh vacuum within and throughout an elongated closed space; (2) providing and maintaining of such beam of charged particles to the point of use thereof at substantially the area" of termination of such substantially uniform electrostatic field; (4) and injecting from asuitable operatorcontrolled source of electrons or ions such a beam of charged particles immediately and continuously into such substantially uni-form .electrosta tic field maintained in such very high vacuum, so that such beam is accelerated in saidsubstantially uniform electrostatic field to its point of 'useywhereby, through the presence and action nuclear physics, cancer therapy; radiography,

high-voltage X-rays, cathode rays, etc., so that all the particles of the beam are delivered as a beam of charged, swift particles to a small focal point with the particles all having the same energy and traveling in parallel paths, comprising the'following steps: (1) providing and maintaining a very high vacuum within an enclosed, elongated space having conducting surfaces at its ends, at least; (2) providing and maintaining throughout the transverse and longitudinal extent of such enclosed space a substantially uniform electrostatic field that is undisturbed and undistorted in the carryin out of the method by electrically associating conducting surfaces thereof with corresponding electrodes of a high voltage generator; (3) injecting continuously from a suitable operator-controlled source of electrons or ions'a beam of charged particles immediately into such substantially uniform electrostatic field, thereby preventing any localized source of ionization by avoiding initial spontaneous breakdown; (4) providing and maintaining a uniform potential gradient throughout said enclosed space from the point of injection of said beam to the point of use thereof at substantially the area of termination of such substantially uniform electrostatic field; (5) causing such beam of charged particlesto be accelerated as a beam of charged swift particles by the action of such substantially uniform electrostatic field; and (6) causing the said beam of accelerated charged swift particles to be delivered to the point of use thereof, with 'allthe said charged swift particles traveling in parallel paths and all with the same generator energy.

.4. That method of generating and accelerating the transmission of a beam of swiftparticles, either electrons or ions, for use in fields of nuclear physics, cancer therapy, radiography, high-voltage X-rays, cathode rays, etc.,'so that. all the particles of the beam are delivered as a beam of charged swift particles to a small focal point with the particles all having the same energy and travcling in parallel paths, comprising the following steps: (1) providing and maintaining a very high vacuum Within an enclosed, elongated, space havin electrode surfaces along the wall thereof; (2-) providing and maintaining throughout substantially the transverse and longitudinal extent of such enclosed space a substantially uniform electrostatic field that is undisturbed and undistorted' in the-carrying out of the method and is created and maintained through the electrical associati'on of said electrode surfaces along the wall of 'said spacew'ith corresponding electrodes of a high'voltage generator, and in so doing leaving open and'unob'structed the region along the axis of the said enclosed, elongated, insulated space wherein the high voltage is to be attained,lso that the injected beam of charged particles can be accelerated as a beam of. charged swift particles .alongthe axis of the enclosedz'space; (3) in;- jecting continuously from 'a suitable operatorcontrolled source of electrons or ions'albeam of charged particles directly into such substantially uniform electrostatic field; (4) by the action of such substantially uniform electrostatic field, causing the undesirable secondaries that are produced by impact of stray, high voltage, swift particles upon such electrode surfaces along the Wall'of said enclosed space,'to travel along lines parallel to the axis of such enclosed space, thereby preventing such secondaries from building up a cumulative breakdown; (5) and'by the action of said substantially uniform electrostatic field causing the said beam of charged-particles to be delivered as a'beam of charged swift particles to the" point of use thereof with all the said charged swift particlestraveling in p'arallelpaths and all with the same generator energy. V

5. The method of effecting the generation of and the accelerated transmission of an accurately focused beamfof charged swift particles for use in fields of nuclear physics, cancer therapy, radiography, high voltage X-r'ays, cathode rays, etc., through the performing of a-series of steps, including the preparation and manipulation of an elongated acceleration tube or like electronic envelope, by physical acts of the operator thereof, comprising: (or) removing thegas by suitable operator action from within said acceleration' tube or electronic envelope to such an extent as to create anextremely high vacuum therein; (12) as a physical act, positioningand electrically associating different and corresponding electrodes of ahigh voltage generator with relation to conducting surfaces of said accelera tion tube or electronic envelope which surfaces are situated at the opposite ends, at least, of said tube or envelope, so as by such physical act to provide surfaces at controlled potentials along the wall of said tube or envelope, the said surfaces at controlled potentials. and which S111? -ro'und the axial region of the tubeor envelope being maintain-ed at proper. relative values of potential by the act of the operator in so electrically positioning and associating. them with relation to corresponding electrodesmaintained at carefully controlled potentials by the operation of the high voltage generator; and in so do ing providing substantially throughout the axial region of the tube or envelope a substantially uniform electrostatic field that is undisturbed and undistorted along the longitudinal axis of such enclosed space of the tube or envelope; and

I :(c) through the employment and manpiulation of a device for producing at the high voltage end of the said tube or envelope (after such creation of an extremely high vacuum therein) charged particles, injecting such particles into the region of a uniform, steady and continuous electric field in such tube or envelope, while preventing adverse effects from such transverse velocity as such charged particles may have, whereby the beam of charged particles so injected is accelerated along the longitudinal axis of such tube or 17 'nvelop ,and the said physically-performed acts of:': the operator through the agency and opera-- tion of: the said substantially uniform electrostatic field, cause: the beam of particles to be delivered asa beam of charged swiftaccelerated particles towthe pointof usethereof at theoppo-- siteend portion. of said tubetor envelope from the point? of injection of such chargedparticles,

with allsuchchargedparticles traveling in parallelpaths and all with the same generator energy.

6.- The t method of eifecting; the generation of and the accelerated transmission: of an accu-r rately focused beam of charged swift particles for use in fields of nuclear physics, cancer therapy, radiography, high voltage X-rays, oath-= Ode rays; etc. through the performing, of aseriesof. steps involving the preparation: and manipulation by'physical acts of the'operator of an elon-- gated acceleration tube or. like electronic envelope, comprising: ('a) removing'the gas by suitable operator-controlled manipulation from within said acceleration tube or electronic envelopei to such an extent as to create an extremely" high vacuum therein; (6) connecting diflerentand corresponding electrodes of a high voltage generator with corresponding conducting surfaces of said acceleration tubeor electronic envelope situated at the opposite ends, at

least, of said tubeor envelope, so as'thereby to provide surfaces at controlled potentials along 4 thewall of the said tubeorenvelope, said sur faces at controlled potentials and surrounding theaxial region or the tube or envelope being maintained at proper relative values of potential by the act of the operator insuitably connectingthem to corresponding electrodes ma ntalned at carefully controlled potentials by the operation of'a high voltagegenerator, and in so doing providing throughout the axial region of' the tube or envelope and throughout at least a prolonged initial portion of the longitudinal extent of such tube or envelope a substantally uniform electrostatiefleld that/is undisturbed" and undistorted along the longitudinal axis, of

suchenclosed space ofthe' tube or envelope; and (c) then through the employment and manipulations of a device for producing at the high voltage end of the said acceleration tube or envelope,

(after the said creation of such: extremely high vacuum therein), charged particles, injecting such charged particles intothe region of a uni- J form, steady and continuous electricfield in said tube or envelope'in' a way; to preventadverse effeet from such. transverse velocities as such charged particles may have; whereby the beam of charged particles so injected is accelerated along the axis of such tube or envelope and there is effected a reduction of the auto-emission or held currents of" electrons at the said surfaces at controlled potentials and surrounding the axial region; of the tube or envelope, and where-' by there is effected a reduction of the secondary emission of charged particles dueto the impact of stray hlghvoltage particlesonsuch surfaces that are at; controlled potentials, and thus by the, said physically performed acts of the operator causing the 'beamoi' charged swift particles to be delivered to the point of use at the opposite end of said tube or envelope from the point of injection of such charged particles, with all such particles traveling in parallel paths and all with the same generator energy.

7. The method"f effecting the generation of and the accelerated transmission of an accurately focused beam of charged swift particles for use in fields of nuclear physics, cancer therapy, midi--- ography, highvoltage X-rays, cathode rays, etc. through theperforming of a series of stepsinvolving the preparation and manipulation, by physical acts of the operator of'anelongated ac-- celeration tubeor like electronic enve10pe, c0m-,' prising: (a) removingthe gas by suitableoperator-controlled manipulation fromwithinsaid ace celeration tube or electronic envelope to such an extent as to create an extremely high vacuum therein; (17) breaking up the voltage along, the; length of ithe tube or envelope into small divisions;

by the, physical act of the operator in associating different electrodes along the wall of the tube; or

envelope with different but, corresponding 6190',"

trodesof: a high voltage generatonso as thereby;

to provide surfaces at controlled potentials along; the wall of the saidtube or envelope, said-surfaces: at controlled potentials and surroundingthe axial region of the tubeor envelope being maintained atproper relative values of, potential by the-act of the operator in suitably associating them with,v

corresponding electrodes maintained at carefully controlled potentials by the operationof a high voltage generator, and in so doing providing throughout the? axial region of the tube or envelope and throughout atleast a prolongedinitiali portion ofthelongitudinal extentof such tube or envelope a substantially uniform electrostatic;

field that is undisturbed and undistorted; along the longitudinal axisof such enclosed space of i the;

tube or envelope; and (c); thenthrough theeeme ployment'and manipulation of a device forv pro-F ducing at the high voltageend of the said acceler-e ation tube orenvelope (after the said creationof;

such-extremely highrvacuum therein), charged:-

particles. injecting, such charged particles-into theregion of a uniform, steady and continuous. electric field in said-tube or envelope in-a way to preventadverse effect from such transverse:

velocities as such chargedparticles may have;

operator causing the beamof charged swift; particles to be delivered to the pointyof useat; the, 1 opposite end of said: tube. or envelope, from; the, point of injection ofsuchcharged:particlesawitha all such particles traveling in parallelpaths and:

all, with the same generator, energy.

8. The method ofeffecting the generationrof'i and. the accelerated transmission of an accurately; focusedbeam of charged swift particleszfor usein a fieldsof nuclear physics, cancer therapy radia ography, high voltage X-rays, cathode, rays,-etc..

through the performing of: a.seriesof.steps involving. the preparation and manipulationiby physical acts of the operator of an elongated acceleration, tubeior like electronic envelopeccom prising: (at) removing, the gas by, suitable: operxif ator-controlled manipulation from said acceleration tube or electronic envelope.it-oiz suclrail an extent as to create an extremely high vacuum therein; (b) connecting different and corresponding electrodes of a high voltage generator with corresponding conducting surfaces of such acceleration tube or electronic envelope, situated at the opposite ends, at least, of said tube or envelope, so

as thereby to provide surfaces at controlled po'- tentials along the wall of the said tube-or envelope, said surfaces at controlled potentials and surrounding the axial region of the tube or envelope being maintained at proper relative values of potential by the act of the operator in suitably connecting them to corresponding electrodes maintained at carefully controlled potentials by the operation of a high voltage generator, but warping somewhat the substantially uniform electric field in the region of the injection of the charged particles, so as properly to counteract any residual tendency of the incoming charged particles toward divergence or convergence, by slightly respectively reducing or increasing the potential gradient in the cylindrical region surrounding the axis of the tube or envelope in the region of the initial acceleration, by the act of the operator in reducing or increasing the voltage differences between the generator electrodes at the end portion of the tube or envelope where the charged particles are produced, to effect the final focusing of a substantially parallel beam of charged, swift particles, and providing throughout the axial region of the tube or envelope and throughout at least a prolonged initial portion of the longitudinal extent of such tube orenvelope a substantially uniform electrostatic field that is undisturbed and undistorted along the longitudinal axis of such enclosed space of the tube or envelope; and (c) then through the employment and manipulation of a device for producing at the high voltage end of the said acceleration tube or envelope (after the said creation of such extremely high vacuum therein), charged particles, injecting such charged particles into the region of a uniform steady and continuous electric field in said tube or envelope in a way to prevent adverse effect from such transverse velocities as such charged particles may have; whereby the beam of charged particles so injected is accelerated along the axis of such tube or envelope, and there'is efiected a reduction of the auto-emission or field currents of electrons at the said surfaces at controlled potentials and surrounding the axial region of the tube or envelope, and whereby there is effected a reduction of the secondary emission of charged particles due to the impact of stray high voltage particles on such surfaces that are at controlled potentials, and thus by the said physically performed acts of the operator causing the beam of charged swift particles to be delivered to the point of use at the opposite end of said tube or envelope from the point of injection of such charged swift particles, with all such particles traveling in parallel paths and all with the same generator energy.

9. A method in accordance with claim 6, but wherein, as a step thereof, the operator connects numerous electrodes that are postioned successively along the wall of the acceleration tube or likeelectronic envelope with corresponding eleclengthwise wall of the tube or envelope, a substantially. uniform electrostatic field is established along substantially the entire length of such acceleration tube or like electronic envelope.

"11'; A method iniaccordance with Telaim 7,Ibut' wherein, as a step'thereof, the operator connects numerous electrodes that are located successively.

along the lengthwise extending wall of the acceleration tube or like electronic envelope with corresponding electrodes of a generator, and in so doing he thereby provides a uniform potential gradient throughout the enclosed space of the said tube or envelope from the point at which the charged particles are injectedfinto said tube or envelope to substantiallythe point of use thereof at the opposite end envelope.

,12. A method in accordance with claim 6, 'but' wherein, asa' step of the method, the so-provided substantially uniform electrostatic field imparts to the charged particles 11 large velocity. accurately in the axial direction of thetube or envelope without their acquiring at the same time an undesired transverse velocity, whereby the saidbeam of charged swift particles is accurately focused. Y

13. A method in accordance with claim 6, but wherein the said device for producing chargedj swift particles is so positioned and is so manipulated by the operator as to inject the said beam of charged particles immediately into the said substantially uniform electrostatic field, whereby the said method prevents any localized source of j ionization due to initial spontaneous electrical velope to be in a direction perpendicular to the surfaces of such electrodes, and thereby he effects the holding to a minimum of the produc: tion of secondary particles that contribute to high-voltage breakdown, and there is also effected the acceleration in a direction substantially parallel with the axis of the enclosed space, the

low energy particles from the surfaces of such electrodes along the lengthwise extending wall of the said tube or envelope. 7

ROBERT J. VAN or GRAAFF. WILLIAM WEBER BUECHNER.

' REFERENCES. orrsn.

The following references file of this patent:

UNITED STATES PATENTS I c Number Name Date 2,005,021 Brasch et a1. June 18, 1935 2,145,727 Lloyd, Jr Jan. 31, 1939 2,182,185 3 Trump -1 Dec. 5, 1939 2,264,274 Broadway Dec. 2, 1941 2,336,774 Brackney et al Dec. 14, 1943 2,376,439 Machlett'et al. May 22, 1945 2,433,682 Bradley Dec. 30, 1947 2,460,201 Trump et al Jan. 25, 1949 2,517,260 Van de Graaff et al. Aug; 1, 1950' portion of the said tube or are of record in the 

